docs: def complement(self):
          """Return the complementary relationship of self.
  
          Raises:
              ValueError: if the relationship has a complementary
                  which was not defined.
  
          Returns:
              complementary (Relationship): the complementary relationship.
  
          Example:
  
              >>> from pronto.relationship import Relationship
              >>> print(Relationship('has_part').complement())
              Relationship('part_of')
              >>> print(Relationship('has_units').complement())
              None
  
          """
          if self.complementary:
  
              #if self.complementary in self._instances.keys():
              try:
                  return self._instances[self.complementary]
              except KeyError:
                  raise ValueError('{} has a complementary but it was not defined !')
  
          else:
              return None